Its difficult to say really - that would mean that Fiverr would be getting into quality control on gigs and there are probably a lot of more obvious targets for that first. The subscribers, followers etc are already not allowed as they are against third party ToS. With links and traffic it is more complicated.
With Traffic, it really depends on the method of “sending traffic”. For example, using pop-up and pop-under ads CAN actually be quite successful. It is generally real people getting your website popped up in front of them. If you site is eyecatching then it is entirely possible that they might click and engage with it, perhaps even buy or whatever the point of the site is.
Im sure there are other methods that work too but I used to have a site which sold pop-under traffic. I eventually quit, not because of the service but because of the clientele. The majority were awful, ugly, spam and virus infested messes which people closed immediately for obvious reasons and therefore the client came back complaining that the traffic was bad! It was simply not worth my time.
Links - I do SEO consultations and a lot of my work is problem solving for sites that have bad search rankings or seen a drop in traffic and they dont know why. MY FIRST QUESTION is always - can you send me the reports and the names of anyone who has done SEO on your site in the last 12 months. I don’t look at anything until I have checked that as I would say 60-70% of the time, it is down to bad SEO, links etc - ie. Caused by the SEO service that they had to pay for!
This is particularly difficult for long existing and popular sites who have thousands of genuine and good quality links built up over years and fixing it is a painstaking process which obviously costs money as you cant simply just wipe all of their links. If a new site buys a ton of links then sure, you can clear it in 10 minutes usually.
Of course, buying links is not always bad but there are criteria which have to be met. Like anything else, it is up to the buyer to really check out what they are buying. A Google search about link buying will give a huge amount of info so there isn’t any excuse for people to not check it out.